How to Re-download ML if it's already installed

I want to make a USB ML install drive but my RMBP came already with ML, and since it was already installed I can not re-download it from App store, it wouldn't show it my previous purchases. I searched the forum but couldn't find a solution. Do you guys know how I can download it so I can put it on USB drive for quicker install? Thanks!

Thank you for reply but it's not working. Like I mentioned before there is no option to Re-Download it since I've never purchased it in first place, it would not show in purchases history. It was already installed when I bought the computer, so I can not see it in my previous purchases.

Sorry to give the wrong info.
go to an apple store or call 800 apple number, see if they can help you.
I understand if you go to apple store (brick and mortar store) along with your macbook pro and a sd card or usb drive 8gb one, you can get an ML osx physically in your hand.

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